This invention relates to power supplies, particularly to regulation of DC converter power supplies, and more particularly to power supply switching circuits that have binary feedback regulation.
In the prior art, DC converters generally utilize pulsed DC power to the primary winding of a transformer in the converter, and the voltage on individual transformer secondary windings is rectified and filtered back to a DC output voltage.
Such DC converters provide a higher power density and increased power throughput than previous power supply technology, partly due to the higher frequency switching rate for the DC voltage applied to the primary winding of the converter transformer. The switching frequency of existing DC converters is in the range of 50-100 KHz. However, increasing the switching frequency of converters beyond the frequency presently used in the art is not practical to achieve greater power density. The converters exceed threshold switching frequencies and tend to become unstable. The instability is usually caused by variations in the electrical load or inadequate phase margin and results in instability in the output voltage level.
Conventionally, a single feedback loop from one of the DC outputs is used as the feedback to control the pulsed DC power delivered to the primary winding of the transformer in order to regulate the DC converter output. This control feedback is commonly an analog, integrated, error signal which is used to control the pulse width of DC pulses input to the converter transformer primary winding in order to maintain the desired output voltage. The feedback can also be binary as described in U.S. Pat. No. 5,260,861 xe2x80x9cDigital Controlled Converter and Methodxe2x80x9d by Harry E. Wert, Nov. 9, 1993.
The shortcoming in the prior art is that the voltage present at DC converter transformer secondary windings is unregulated, except for the voltage output from the one secondary winding that provides the feedback control signal to the circuitry that controls the width of DC pulses input to the converter transformer primary winding. These unregulated outputs are often referred to as xe2x80x9cquasi regulatedxe2x80x9d since there is some degree of regulation against variations in the input DC voltage as a result of sensing the variations at the one regulated output. However, a given change in the pulsed DC voltage on the transformer primary will not, usually, effect the output voltage at all secondary windings of the DC converter transformer in the same proportion. This produces either too much or too little compensation to the xe2x80x9cquasi-regulatedxe2x80x9d DC converter outputs. There is no voltage regulation provided for the quasi-regulated outputs due to load changes on those outputs.
This lack of regulation on some outputs can be overcome by providing each quasi-regulated output with its own independent transformer primary winding with feedback control and circuitry that controls the pulse width of DC pulses input to the primary winding. However, this approach is very expensive. In addition, using separate sets of control circuits as just described results in reduced overall efficiency in that the switching losses and transformer losses of the sum of these individual units is greater, for the same total output power.
A more common approach to solving this problem is to add linear regulators to each of the xe2x80x9cquasi-regulatedxe2x80x9d outputs that requires full regulation. While this approach works there is a substantial reduction in operating efficiency due to the dissipative nature of linear regulators.
In addition, when there is a step load change on an output of a prior art DC DC converter there is momentary voltage overshoot or undershoot on the particular output depending on if the load is increasing or decreasing. This is partly due to the fact that in the prior art error signals are averaged over a finite period of time before a correction is made to the output voltage.
Thus, there is a need in the art for improved switching circuitsDC that can provide higher power density, improve stability and smaller packaging.
There is also a need in the prior art for simple regulator circuitry in a DC converter that can provide independent regulation of the voltage output from each of multiple outputs of the converter, even under varying loads at each of the outputs, without requiring dissipative regulation circuitry for each output of a converter.
In addition, there is need for a regulator that can prevent or minimize overshoot and undershoot when there is a step load change on the converter outputs.
The above needs in the prior art and objects of the invention are satisfied or met by the present invention.
The present invention provides an improved switching circuit with improved operating efficiency, higher power density, improved stability, wide loop bandwidth, and smaller packaging than heretofore possible in the art. The invention does this by providing regulation of the voltage output at each of multiple outputs of the converter, even with varying loads at each of the outputs, and can prevent overshoot and undershoot when there is a step load change, while maintaining maximum efficiency under different load currents.
The invention replaces the usual analog or digital feedback error signal from one output of a prior art switching circuit such as used in DC converters, and sometimes voltage regulators associated with others of the outputs, with a binary on/off feedback command from a comparator associated with each output of the converter. The on/off command from a comparator controls the voltage at its associated output, and the on/off commands from each output are combined to create another single on/off feedback command that controls the voltage applied to the primary of the transformer of the converter. The on/off feedback commands only indicate if the voltage at each output of the circuit is above or below a set value. In this manner power dissipative regulator circuits of the prior art are eliminated, and the pulse generator circuitry driving the converter transformer primary is greatly simplified.
This invention is implemented with simple circuitry which reduces parts count and improves the inherent reliability as well as allowing smaller packaging and reduced cost. In addition, the reduced power dissipation of fewer components contributes to smaller size and improved reliability.
More particularly, with the present invention, the voltage at each output is compared to a voltage reference signal for that output by a comparator to obtain a binary error signal which indicates only that the respective output voltage is too high versus too low or in range when compared to the reference signal voltage for that output, and without indicating how much the output voltage is high or low. This binary error signal from each output is used to control a switch that opens the secondary winding of the converter transformer associated with each respective output when the voltage thereat is too high, and controls the switch to leave the secondary winding in the circuit when the output voltage is in range. The in range voltage is defined by the known hysterisis (or dead band) of the error voltage comparator.
The binary error signal from the comparator associated with each output of the converter are combined with a logic gate and used to control the application of a source of energy to the primary winding of the converter transformer. Power is applied to the converter primary winding when at least one binary error signal indicates that its associated output is too low.
In the primary embodiment of the invention the pulsed DC power applied to the converter transformer primary always has a constant frequency and pulse width, unlike prior art DC converters where the frequency or pulse width is changed as part of the regulation. This results in much simpler circuitry.
The binary error signal is used to switch the DC pulses applied to the transformer primary on or off as soon as indicated by the error signal. There is no waiting for the beginning or ending of any pulses. This allows nearly instantaneous converter power change in response to line and/or load variations. Absent is the intentional error delay that conventional converter technology requires to provide loop stability. The binary feedback arrangement of the present invention is inherently stable, without delay or integration, because it is one dimensionalxe2x80x94indicating only that output voltage is in or out of range, and not how much it is in or out of range. This equates to an equivalent loop bandwidth that is equal to the converter switch frequency instead of, typically, ten-percent of the switch frequency for conventional feedback control.
Since regulation is achieved by disallowing or blocking converter pulses in one or more transformer secondary windings, efficiency is enhanced by directing the finite pulse power of each converter cycle to the particular secondary winding that requires power.
This novel operation provides full independent regulation of each output of the DC converter while providing high operating efficiency, ideal transient response, and overall simplicity. Additionally, there are no cross regulation problems as experienced with existing DC converters.
It is a characteristic of the DC converter that the ripple amplitude is essentially constant over the design load range. In the prior art the ripple amplitude is maximum at full load and minimum at minimum load.